Due to advances in polymer science, the number of applications for polymeric tubing have drastically increased. These advances have resulted in polymeric tubing having outstanding thermal, mechanical, and insulative properties. Moreover, due to the wide variety of base resins, additives, and processing techniques currently available, tubing can be designed having specific physical properties, e.g. flexibility, flame resistant, chemical resistant, etc.
Typically, the base resins used for such polymeric tubing include polyolefins, polyvinylchloride (PVC), fluoropolymers, elastomers, and blends thereof.
Depending upon the materials and processing techniques used, such tubing may be "heat-shrinkable". For example, polyolefin materials are commonly extruded, irradiation crosslinked, and then expanded to form heat shrinkable tubing, as is common in the art and as described in: R. Kraus and D. Ryan, "Advances in Heat-Shrink Technology," IEEE Electrical Insulation Magazine (1988) Vol.4, 31-34; and J. W. Hoffman, "Insulation Enhancement with Heat-Shrinkable Components," IEEE Electrical Insulation Magazine (1991) Vol.7, 33-38. Upon subsequent application of heat, such tubing shrinks to approximately its originally extruded size and shape.
Although heat shrinkable polyolefin tubing exhibits many desirable mechanical, thermal, and insulative properties, in many applications including those having repeated exposure to high temperature and/or pressure, such polyolefin materials fail to maintain an adhesive bond to a substrate and/or fail to maintain heat sealability (ability to remain bonded to itself). Consequently, in these applications, an inner sealant or adhesive mastic liner is commonly used to help maintain a fluid tight seal. For example, U.S. Pat. No. 3,297,819 to Wetmore; R. Kraus and D. Ryan, "Advances in Heat-Shrink Technology," IEEE Electrical Insulation Magazine (1988) Vol.4, 31-34; and J. W. Hoffman, "Insulation Enhancement with Heat-Shrinkable Components," IEEE Electrical Insulation Magazine (1991) Vol.7, 33-38 all describe a heat shrinkable tubing including an inner thermoplastic or mastic liner which maintains an adhesive bond with an inner substrate, and an outer heat shrinkable liner which is typically crosslinked. Such tubing may be co-extruded thereby providing a tubing having the inner non-crosslinked liner and the outer crosslinked liner.
Many problems are associated with such tubing, particularly when used in environments having cyclic exposure to high temperatures and pressures. For example, when such tubing is used in the medical or food industries where cyclic autoclave sterilization is common, the inner liner of the tubing melts and flows in response to the sterilization temperatures and pressures. Moreover, at sterilization temperatures, the outer liner tends to undergo additional shrink thereby causing the melted inner liner to ooze or flow from the tubing. The melt and flow of the inner liner, although acceptable in some applications, is not acceptable in most medical applications. For example, one particular medical applications includes the use of the heat shrinkable tubing in connection with electro-surgical laparoscopic instruments. Such instruments typically include a cylindrical electrically conducting member having one of many possible surgical attachments secured to one end for performing a variety of surgical procedures, i.e. providing suction, irrigation, coagulating vessels, etc. The opposite end of the conducting member is securable to a hand-held control module which allows a surgeon to control the surgical attachment.
The conducting member includes a sheath or tubing disposed circumferencally about its length for electrically insulating the conducting member. The tubing is preferably transparent and of the heat shrink variety so that it may be easily applied about the conducting member. Moreover, the tubing must maintain an adhesive seal with the conducting member and maintain heat sealability with itself after cyclic autoclave sterilization in order to prevent the ingress of moisture between the conducting member and the tubing. The tubing must maintain its electrical insulating properties along with a sufficient hot modulus. Furthermore, the tubing must maintain all of these aforementioned properties after cyclic gamma, electron beam, or ethylene oxide gas sterilization procedures. Finally, the tubing must maintain its thermal stability at sterilization temperatures and not flow in response to sterilization heat and pressure.
Prior art heat shrink polyolefin tubing, including those made from KYRAN.TM. (a registered trademark of Penwalt Co. for its vinylidene fluoride resin) and crosslinked polyolefins such as those disclosed in U.S. Pat. Nos. 3,592,881 to Ostapchenko, 3,990,479 to Stine et al., and 3,852,177 to Atchison et al., may provide the necessary hot modulus strength, but lose their adhesive sealability to a substrate and lost sealability after exposure to the high temperatures and pressures associated with autoclave sterilization. Moisture leaks into the interface between the conducting member and the tubing, thus causing potential electrical and sterilization problems. Such prior art tubing also permanently discolor after cyclic gamma sterilization. Furthermore, the prior art tubing lose their electrical insulating properties after cyclic sterilization. Although co-extruded tubing such as that shown in U.S. Pat. No. 3,297,819 to Wetmore, may maintain a seal after repetitive autoclave sterilizations, the temperature and pressure associated with sterilization cause the inner liner of adhesive material to flow out of the tubing rendering the instrument unusable and susceptible to leaks.
Thus, a tubing is needed which is thermally stable and will not flow at sterilizations temperatures while simultaneously providing sufficient hot modulus, electrical insulative properties, permanent transparency, heat sealability, and adhesiveness after cyclic autoclave, gamma, and electron beam sterilization.